When a noni fruit ripens, it stinks like old cheese—or even vomit. Familiar to many in the form of expensive juices sold as health supplements, this pungent fruit is engaged in a slow-motion arms race with would-be insect pests. Fruit flies are unable to feast on noni—scientific name Morinda citrifolia—because the fruit is dosed with large quantities of octanoic acid (OA), making it highly toxic to them. But one species has sidestepped this defense system: in tropical Seychelles, a specialized type of fruit fly called Drosophila sechellia has evolved to feed exclusively on the unappetizing fruit.

To learn how D. sechellia thrives on a diet that should kill it, Lanno et al. looked for D. sechellia genes that were differentially expressed when the flies were given OA-containing food. Their analysis revealed 104 differentially regulated genes that have known orthologs in the OA-susceptible fly D. melanogaster. Many of the downregulated genes are involved in the immune system—an interesting result given the recent finding that D. sechellia can’t mount an immune response to an attack by the parasitoid wasp Asobara tabida. The downregulated immune genes include some that are involved in responses to a variety of threats, including bacteria, so OA exposure may deal a major blow to the immune system.

Among the upregulated genes are several in the insect-specific Osiris family. One of these, Osi6, is the only gene in the set of 104 that is found in a region of the genome known to have a major impact on OA resistance. This aligns with the research group’s previous discoveries that Osi6 expression is 72 times greater in D. sechellia than it is in D. simulans, an OA-susceptible fly, and that knocking down Osi6 in D. melanogaster makes it even more vulnerable to OA.

A different research group recently found that the Osiris gene cluster is under strong selection in an isolated population of the fly D. yakuba that has just begun adapting to a diet of poison-laden noni, another clue that learning more about these genes may be crucial for understanding OA resistance and this compelling model of ecological adaptation.